1. Field of the Invention
The present invention relates to a wheel motor drive system for electric automobiles, and more specifically to a drive system which is capable of providing an energy-saving driving mode for drive motors connected to vehicle wheels.
2. Description of the Background Disclosure
In recent years, there have been proposed and developed various electric automobiles employing at least one electric motor. Such electric automobiles have been disclosed in Japanese Patent First Publications Tokkai (Showa) 56-132102 and 62-138002, the former being a hybrid driving system in which both an internal combustion engine and an electric motor are provided for driving vehicle wheels, and the latter being a wheel motor drive system in which two same motors are connected to left and right wheels for driving these wheels independently of each other. In the former system, the wheels are driven by the internal combustion engine within a particular driving torque range wherein a relatively high combustion efficiency of the engine is obtained, and also driven by the electric motor within the other torque range. In the latter system, the left and right motors are controlled independently of each other, such that both motors are simultaneously driven with the same rotational speed during a straight driving of the vehicle, and the rotational speed of the outer wheel is set to be greater than that of the inner wheel on the basis of a monitored steering angle and rotational speeds of motors during a left or right turn, with the result that the outer wheel travels farther than the inner wheel. Such a motor driving control for the wheel motor drive system functions as a differential gear which allows vehicle wheels to rotate by different amounts when the vehicle rounds a turn.
In the previously noted Japanese Patent First Publication No. 56-132102, since one of the internal combustion engine and the motor, which are connected to each other through a one-way clutch, is selected to drive the wheels in consideration of a desired high combustion efficiency of the engine, there is a possibility that the wheels would be driven by the motor even at a relatively high torque range wherein the engine exhibits a low combustion efficiency. To permit this, a high-power motor is required. As is well known, an efficiency of an electric motor is relatively low, such as 60% or less, when the rotational speed of the motor is low or when an output torque required for the motor is low. Therefore, as the maximum output torque of the motor is greater, the efficiency of the motor is deteriorated during a low-load travelling of the vehicle, such as a constant speed travelling, since the difference between the maximum torque and the required torque becomes greater. This results in a relatively short travel distance of the vehicle in one discharge cycle according to which the car battery condition is changed from a fully charged state to a completely discharged state. Under these constructions, if the vehicle is held at a low vehicle speed so as to lower a travel resistance thereof, the output torque required for the motor becomes lower, with the result that the above torque difference becomes greater and the motor efficiency is considerably lowered.
In the above mentioned Japanese Patent First Publication No. 62-138002, there is a drawback of providing a relatively low motor efficiency during the low-load travelling, since the identical, left and right motors are utilized and simultaneously driven at the same rotational speed during a straight driving of the vehicle.